1. The three-dimensional ultrastructure of endothelial intercellular clefts of frog mesenteric capillaries of known hydraulic permeability (Lp) has been investigated in the absence and presence of lanthanum ions as tracers of extracellular solute. 2. Experiments were carried out on the exposed mesenteries of pithed frogs and Lp of a chosen microvessel perfused with a Ringer solution containing serum albumin (10-40 mg ml-1) was determined. In some experiments the mesentery was fixed in situ with 2.5% glutaraldehyde immediately after Lp had been measured. In other experiments, measurement of Lp was followed by brief microperfusion (10-20 s) with a second Ringer solution containing 1% lanthanum nitrate as a tracer before in situ fixation of the tissue. The tissue was prepared for electron microscopy using standard techniques. The perfused capillary was identified in the block and serial transverse sections were cut along its length over regions where Lp had been measured. 3. In six capillaries where the tissues were fixed immediately after measurement of Lp, Lp had a mean value (+/- S.E.M.) of 4 (+/- 0.5) x 10(-7) cm s-1 (cmH2O)-1. Serial (30-40 nm) sections of these vessels revealed that a single short narrow region of the intercellular clefts ran almost continuously from section to section. Additional tight regions were regularly seen, but they usually extended for relatively few sections. In 13.36 microns of reconstructed cleft, there were three interruptions of the tight region of 0.14, 0.14 and 0.17 microns respectively. In the region of these discontinuities, the wide region was uninterrupted from luminal to abluminal surface. 4. Examination of the tight junction on a tilting stage revealed that the outer leaflets of the adjacent cells were not fused, but separated by a gap of mean width (+/- S.E.M.) 2.3 (+/- 0.1) nm. 5. In four capillaries perfused with lanthanum nitrate before fixation, mean Lp (+/- S.E.M.) was 6.5 (+/- 0.02) x 10(-7) cm s-1 (cmH2O)-1. Segments of intercellular clefts, totalling 23.56 microns in length, were reconstructed from serial sections and throughout these, electron-dense deposits of lanthanum were observed to fill the luminal parts of the intercellular clefts up to the tight region. Lanthanum deposits filled the entire cleft to the abluminal surface at eleven sites, which accounted for a length of 2.52 microns out of the 23.56 microns. Only five of these regions were delimited within a continuous series of sections and their mean length (+/- S.E.M.) was 0.16 (+/- 0.063) microns.(ABSTRACT TRUNCATED AT 400 WORDS)